Bird ID Skills: Size & Shape

Birds are built for what they do.

Every part of the bird you're looking at is a clue to what it is.

Henslow's Sparrow Photo by Luke Seitz

April 20, 2009

The combination of size and shape is one of the most powerful tools to identification. Though you may be drawn to watching birds because of their wonderful colors or fascinating behavior, when it comes to making identifications, size and shape are the first pieces of information you should examine.

With just a little practice and observation, you’ll find that differences in size and shape will jump out at you. The first steps are to learn typical bird silhouettes, find reliable ways to gauge the size of a bird, and notice differences in telltale parts of a bird such as the bill, wings, and tail.

Soon, you’ll know the difference between Red-winged Blackbirds and European Starlings while they’re still in flight, and be able to identify a Red-tailed Hawk or Turkey Vulture without taking your eyes off the road.

American Tree Sparrow. Sparrows are roughly the same size as warblers but tend to look plumper, and their bills are much shorter, thicker, and more powerful.

American Tree Sparrow. Click forward for another silhouette.

Often you don’t need to see any color at all to know what kind of bird you’re looking at. Silhouettes quickly tell you a bird’s size, proportions, and posture, and quickly rule out many groups of birds – even ones of nearly identical overall size. Practice the silhouettes in the carousel at right.

Silhouettes are so useful because they help with the first step in any identification: deciding what kind of bird you’ve got. Once that’s done, you’ve narrowed down your choices to one small section of your field guide.

Beginning bird watchers often get sidetracked by a bird’s bright colors, only to be frustrated when they search through their field guide. Finches, for example, can be red, yellow, blue, brown, or green – but they’re always shaped like finches. Learn silhouettes, and you’ll always be close to an ID.

Size can be helpful when you compare an unknown bird with a familiar one. A yellow-and-black finch that's smaller than a House Sparrow is probably an American Goldfinch. Evening Grosbeaks look similar, but are almost the size of a robin. Images by John Schmitt/Cornell Lab.

Woodpeckers range in size from the big-as-a-crow Pileated Woodpecker to the Downy Woodpecker, which is barely larger than a sparrow.

Sometimes you need two reference birds for comparison. A Cedar Waxwing is bigger than a sparrow but smaller than a robin. A Blue Jay is larger than a robin but smaller than a crow.

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Judge size against birds you know well

Size is trickier to judge than shape. You never know how far away a bird is or how big that nearby rock or tree limb really is. Throw in fluffed-up or hunkered-down birds and it’s easy to get fooled. But with a few tricks, you can still use size as an ID key.

Compare your mystery bird to a bird you know well. It helps just to know that your bird is larger or smaller than a sparrow, a robin, or a crow, and it may help you choose between two similar species, such as Downy and Hairy woodpeckers or Sharp-shinned and Cooper’s hawks.

Judge against birds in the same field of view

Your estimate of size gets much more accurate if you can compare one bird directly against another. When you find groups of different species, you can use the ones you recognize to sort out the ones you don’t.

For instance, if you’re looking at a gull you don’t recognize, it’s a start to notice that it’s larger than a more familiar bird, such as a Ring-billed Gull, that’s standing right next to it. For some groups of birds, including shorebirds, seabirds, and waterfowl, using a known bird as a ruler is a crucial identification technique.

Size and shape can be very useful for birds in flight, such as mixed flocks of grackles, blackbirds, cowbirds, and starlings. In this photo, look for short-tailed, sharp-winged European Starlings among the large, long-tailed Common Grackles. Photo by Robert Baker/PFW.

A mixed group of gulls can be a real advantage. Here, the enormous Great Black-backed Gulls make the few Herring Gulls behind them seem almost dainty. You almost don’t even notice the rarity, a tiny Black-headed Gull from Europe, in the front row. Photo by Byard Miller via Birdshare.

Use size and shape to find the full range of species hiding in a large flock. Amid these orange-billed Royal Terns are some smaller Sandwich Terns. You’ll also notice a giant Herring Gull in the background, as well as several smaller Laughing Gulls to its right. Photo by Shorebird 777 via Birdshare.

Learn to recognize a few familiar birds, and then use their sizes to measure birds you don’t know. These three common shorebirds—the colorful Ruddy Turnstone, tall Willet, and tiny Sanderling—are a great place to start. Photo by Byard Miller via Birdshare.

Apply your size & shape skills to the parts of a bird

After you’ve taken note of a bird’s overall size and shape, there’s still plenty of room to hone your identification. Turn your attention to the size and shape of individual body parts. Here you’ll find clues to how the bird lives its life: what it eats, how it flies, and where it lives.

Let’s start with an easy one. The pelican’s nearly foot-long fish net of a bill is a good reminder of the dazzling variety of bill shapes in the bird world. Brown Pelican by Birdmandea via Birdshare.

Most woodpecker bills are straight, strong, and sharp, helping the birds drill into wood and pry apart bark. Red-bellied Woodpecker by William Jobes via Birdshare.

Flycatchers have broad, flat bills surrounded by bristle-like feathers that help them catch insects on the wing. Eastern Phoebe by Tripp Davenport via Birdshare.

Vireos are small birds seen among leaves and tree branches. They often look like warblers, but their bills are thicker and very slightly hooked. Red-eyed Vireo by Byard Miller via Birdshare.

Warblers have straight, slender, pointed bills that they use to grab caterpillars and other insects from leaves and branches. Yellow Warbler by Kelly Colgan Azar via Birdshare.

Any bird with a short, thick-based, conical bill spends a lot of its time cracking and eating seeds. Sparrows have moderately sized seed-eating bills. Chipping Sparrow by Michael Hogan via Birdshare.

Grosbeaks have extremely heavy and powerful beaks that make short work of hard-shelled seeds. They are famous among bird banders for giving painful bites. Rose-breasted Grosbeak by Eric T. Black via Birdshare.

Crossbills slip their curious-shaped bills into closed pine cones. As the bird opens its bill, the tips pry apart the cone’s scales, allowing the tongue to dart out and grab a seed. Red Crossbill by katnor1 via Birdshare.

Start with the bill – that all-purpose tool that functions as a bird’s hands, pliers, knitting needles, knife-and-fork, and bullhorn. A flycatcher’s broad, flat, bug-snatching bill looks very different from the thick, conical nut-smasher of a finch. Notice the slightly downcurved bills of the Northern Flickers in your backyard. That’s an unusual shape for a woodpecker’s bill, but perfect for a bird that digs into the ground after ants, as flickers often do.

Bills are an invaluable clue to identification – but tail shape and wing shape are important, too. Even subtle differences in head shape, neck length, and body shape can all yield useful insights if you study them carefully.

Noticing details like these can help you avoid classic identification mistakes. An Ovenbird is a common eastern warbler that has tricked many a bird watcher into thinking it’s a thrush. The field marks are certainly thrush-like: warm brown above, strongly streaked below; even a crisp white eyering. But look at overall shape and size rather than field marks, and you’ll see the body plan of a warbler: plump, compact body, short tail and wings, thin, pointed, insect-grabbing bill.

Measure the bird against itself

This is the most powerful way to use a bird’s size for identification. It’s hard to judge a lone bird’s size, and an unusual posture can make shape hard to interpret. But you can always measure key body parts – wings, bill, tail, legs – against the bird itself.

Look for details like how long the bird’s bill is relative to the head – a great way to tell apart Downy and Hairy woodpeckers as well as Greater and Lesser yellowlegs, but useful with other confusing species, too. Judging how big the head is compared to the rest of the body helps with separating Cooper’s Hawks from Sharp-shinned Hawks in flight.

Get in the habit of using the bird itself as a ruler, and you’ll be amazed at how much information you can glean from each view. Good places to start include noting how long the legs are; how long the neck is; how far the tail extends past the body; and how far the primary feathers of the wing end along the tail (or past the tail).

Downy and Hairy woodpeckers look almost identical and occur in similar habitats. One of the best ways to tell them apart is to judge the length of the bill compared to the head. The Downy Woodpecker's measures only about half the length of its head. The Hairy's is about the same length as the head.

The two common accipiters of North America, Sharp-shinned and Cooper's Hawks, are hard to tell apart. Try comparing the head to the body. Sharp-shinned's head barely protrudes in front of the wings. The slightly larger Cooper's Hawk has a much more prominent head. Images by John Schmitt/Cornell Lab.